Technology update

Jul 28, 2004

Nanoimprint lithography gets smaller

Researchers at Princeton University, US, have shown that photocurable nanoimprint lithography (P-NIL) can produce lines of polymer resist just 7 nm wide with a pitch (or pattern repeat) of only 14 nm. The technique also produced reliable results over the whole area of a 4 inch wafer.

"This work really pushes the limit down to a few molecules in size," said Stephen Chou of Princeton.

During nanoimprint lithography scientists press a mould into a photocurable resist liquid. Exposing the resist to ultraviolet light cures the polymer so that it retains its shape when the mould is removed. Finally, an anisotropic reactive ion etch treatment removes the remaining resist in the compressed area and exposes selected areas of the substrate surface ready for further treatment.

To date, the standard method for making the moulds used in nanoimprint lithography has been electron-beam lithography. This has created patterns of 10 nm dots with a 40 nm pitch but struggles to create patterning with a pitch smaller than 35 nm.

To achieve their record-breaking line pitch result, the team used molecular-beam epitaxy to make the mould. They created a superlattice of alternating layers of GaAs and Al0.7Ga0.3As, and then selectively etched away the Al0.7Ga0.3As layers with hydrofluoric acid. The result was a GaAs mould containing ridges with vertical and smooth sidewalls in a pattern repeated every 14 nm.

"This mould-making process, though time-consuming, would need to be done only once in setting up a manufacturing process," said Chou. "Once the mould is made it can be used to make countless copies very rapidly."

The scientists believe they can make lines in the resist narrower than 7 nm but were unable to examine them in the scanning electron microscope because of thermal damage to the structures from the microscope's electron beam.

Since publishing their work in Applied Physics Letters, the scientists have reduced the pitch between ridges from 14 to 12 nm. According to Chou, this is a 20-fold reduction compared with the state-of-the-art techniques used in making today's most advanced computer chips and would result in 400 times more memory in a two-dimensional memory chip.

The team also used nanoimprint lithography to make gold contacts with a contact gap of just 5 nm. In this case they used a silicon mould fabricated by electron-beam lithography as this is good for patterning small sparse features. "Fabrication of sub-10 nm contacts of sufficient resolution to trap complex macromolecules will substantially advance the emerging field of single-molecule devices by allowing the rapid production of multiple contacts in parallel," the scientists said in their paper.